1. Field of the Invention
The present invention relates to peroxyketals; more specifically to a class of peroxyketals derived from cyclic xcex2-keto esters. Among other things, the peroxyketals of the present invention provide for improved curing of various resins, such as polyesters, than heretofore obtainable. In another aspect, the invention relates to the curing of resins using such peroxyketals.
2. Description of the Prior Art
Organic peroxides have multifarious commercial applications, many of which predominate in the polymer industry. There, organic peroxides are used inter alia to cure various resins, such as unsaturated polyesters, and epoxy vinyl esters; and to crosslink homo- and co-polymers of ethylene and various rubbers. They are also used to polymerize vinyl monomers such as vinyl chloride, styrene, etc.
It is necessary for organic peroxides having uses in commercial and industrial applications in the polymerization and copolymerization of polyesters to be readily reactable and of low cost. The more cost effective the organic peroxide the more applicable the material will be to the industry. One means of decreasing the cost associated with such a material is by decreasing the resin cure time, thus enabling more polymerizations to be conducted over a given period of time.
Peroxyketals have been the subject of several disclosures in the prior art. For example, U.S. Pat. No. 2,455,569 first disclosed peroxyketals obtained from the reaction of tertiary organic hydroperoxides and carbonyl-containing compounds.
2,2-Bis(tertiary butylperoxy)butane is disclosed in U.S. Pat. No. 2,650,913 to be a catalyst for the polymerization of ethylene.
U.S. Pat. Nos. 2,656,334 and 2,692,260 report the use of 2,2-di-(tertiarybutylperoxy)butane as one part of a two component catalyst system useful for polymerization of vinyl compounds.
A series of patents (Great Britain 1,047,830; U.S. Pat. Nos. 4,052,543; 4,328,360; 4,365,086) disclose compounds such as n-butyl 4,4-bis(t-butylperoxy)valerate and ethyl 3,3-bis(t-butylperoxy)butyrate.
U.S. Pat. No. 3,296,184 discloses a series of peroxyketals used to vulcanize polymers and copolymers.
U.S. Pat. No. 3,344,125 discloses a series of substituted cyclohexane peroxyketals.
Esters of 4,4-ditert-butylperoxy pentanoic acid are reported in U.S. Pat. No. 3,433,825 as useful for the crosslinking of copolymers of ethylene and monomers containing terminal vinyl groups.
U.S. Pat. No. 3,763,275 discloses the polymerization of ethylenically unsaturated monomers and the curing of unsaturated polyester resins and elastomers using xcex2-substituted diperoxyketals.
U.S. Pat. No. 4,032,596 reports the use of quaternary ammonium salts as cure accelerators for unsaturated polyester resins containing peroxyketal initiators.
Notwithstanding the teachings of the peroxide materials reported by these printed publications, the prior art is deficient with respect to improved gel times and amelioration of exothermic behavior. Clearly there still remains a need for providing new and improved organic peroxides for use in the polymerization and copolymerization of polyesters. Moreover, there is a need for providing a cost effective organic peroxide which is both thermally stable and which increases the speed with which it polymerizes or copolymerizes a material being acted upon. Such an organic peroxide preferably decreases the gel time of resins which it acts upon and may preferably decrease the exothermic temperature of the reaction as a whole.
The present invention is directed to a class of peroxyketals derived from cyclic xcex2-keto esters and the use of such peroxyketals in the curing of unsaturated polyester resins. Particularly the present invention provides a peroxyketal comprising the general structure: 
wherein;
R1 is C1-C10 alkyl, cycloalkyl, alkaryl, or aryl which may be unsubstituted or substituted with one or more halogen or hydroxy;
R2 and R3 are each independently C1-C10 alkyl, cycloalkyl, or aralkyl; and
n is 1,2 or 3.
As will be made clearer herein, the peroxyketals of the instant invention unexpectedly manifest significantly reduced gel times as compared to the organic peroxides of the prior art. Reduced gel time in this regard will be appreciated as enabling an increased speed of polymerization or copolymerization.
In accordance with another aspect of the present invention, a method of curing a resin is provided wherein a curable cross-linkable resin is contacted with a cure-effective amount of a peroxyketal having the structure: 
wherein;
R1 is C1-C10 alkyl, cycloalkyl, alkaryl, or aryl which may be unsubstituted or substituted with one or more halogen or hydroxy;
R2 and R3 are each independently C1-C10 alkyl, cycloalkyl, or aralkyl; and
n is 1, 2 or 3.
Another benefit provided by the practice of the present invention is a lessening of the exothermic nature of the underlying polymerization reactions which may be found useful in some applications, compared to the prior art. Beneficially, when the curing reaction generates less heat overall, there is a lessening of the development of internal stresses in the cured resin. This in turn will lessen the likelihood of the cured part developing cracks. Product cracking occurs when the heat of exotherm cannot be dissipated satisfactorily, typically during the heating or cooling processes. Product cracking in polymerized and copolymerized reactions potentially results in a total loss of product.
As stated above, the present invention is directed to a class of peroxyketals derived from xcex2-keto esters. The peroxyketals of the present invention comprising the general structure: 
wherein;
R1 is C1-C10 alkyl, cycloalkyl, alkaryl, or aryl which may be unsubstituted or substituted with one or more halogen or hydroxy;
R2 and R3 are each independently C1-C10 alkyl, cycloalkyl, or aralkyl; and
n is 1, 2 or 3.
The substituents referred to are defined herein as follows:
The C1-C10 alkyl group may be either normal or branched; preferably R1 is of the lower alkyl groups in the range from C1-C5, such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, t-butyl, t-amyl, pentyl, and the like.
The cycloalkyl group is C5-C7, preferably cyclohexane (C6).
Each aryl group consists of one or more aromatic rings of C6-C10, including phenyl (C6) and xcex1- and xcex2-naphthyl (C10). Each aryl group may be unsubstituted or substituted with one or more halogen or hydroxy groups.
Each alkaryl group contains up to twenty carbon atoms where each aryl group is C6-C10 substituted with one or more C1-C10 alkyl groups. Preferably the aryl group is phenyl (C6). Each alkyl group is in the normal or branched configuration and is preferably C1-C3.
Each aralkyl group contains up to twenty carbon atoms where each alkyl group is C1-C10 and is substituted with one or more aryl groups; each alkyl group is in the normal or branched configuration and is preferably C1-C4; C3 being most preferred. Each aryl group is C6-C10. In addition, each aryl group may be unsubstituted or substituted with one or more alkyl groups of C1-C3; a preferred aryl is phenyl (C6). A preferred aralkyl is cumyl.
In a first preferred embodiment of the invention, R1 is methyl; in a second preferred embodiment, R1 is ethyl. Preferably, R2 and R3 are each independently C3-C8 alkyls, more preferably they are each independently C3-C6 alkyls, such as propyl, isopropyl, butyl, isobutyl, t-butyl, t-amyl, pentyl, hexyl, and the like. In a preferred practice, R2 and R3 are each independently branched alkyls, including more preferably t-butyl or t-amyl. In a most preferred embodiment, R2 and R3 are the same substituents, e.g. R2 and R3 each are t-butyl or t-amyl. In a preferred practice, n is 1 or 2. In yet another preferred embodiment, R2 and R3 are both aralkyls. Preferably the aralkyls are cumyl groups.
Specific examples of peroxyketals contemplated by the present invention include, but are not limited to, ethyl 2,2-di(tertiary-butylperoxy)-1-cyclohexane carboxylate, ethyl 2,2-di(tertiary-butylperoxy)-1-cyclopentane carboxylate, ethyl 2,2-di(tertiary-amylperoxy)-1-cyclohexane carboxylate, ethyl 2,2-di(tertiary-amylperoxy)-1-cyclopentane carboxylate, methyl 2,2-di(tertiary-butylperoxy)-1-cyclopentane carboxylate, methyl 2,2-di(tertiary-butylperoxy)-1-cycloheptane carboxylate, and the like, for example.
In another aspect, the present invention provides a method of curing a resin comprising contacting a curable or cross-linkable resin with a cure-effective amount of a peroxyketal having the structure: 
wherein;
R1 is C1-C10 alkyl, cycloalkyl, alkaryl, or aryl which may be unsubstituted or substituted with one or more halogen or hydroxy;
R2 and R3 are each independently C1-C10 alkyl, cycloalkyl, or aralkyl; and
n is 1, 2 or 3.
Without limitation, curable or cross-linkable resins contemplated by the method of the invention include, for example, unsaturated polyesters including orthophthalic resins, isophthalic resins, dicyclopentadiene resins; epoxy vinyl ester resins including epoxy novolac-based vinyl ester resins; elastomers including ethylene-propylene copolymers; thermoplastics such as polyethylene; and rubbers such as silicone rubber and styrene-butadiene rubber; or the like.
In the practice of this invention the peroxyketals described aforesaid may be utilized in cure effective amounts. While not being limited to particular quantities, a cure effective amount is determinable relative to the subject resin. Exemplary cure effective amounts are within the range of about 0.5 to about 3.0 peroxyketal parts per hundred parts of resin (phr); preferably about 1.0 to about 1.5 phr.
The peroxyketals of the present invention may be prepared as physical mixtures in the form of liquids, granules, powders or flakes. A physical mixture in accordance with the present invention may be prepared by mixing a peroxyketal with the desired amount of solvent to retain reactivity and usefulness in polymerizations and copolymerizations.
Suitable organic solvents include those normally employed for such peroxides, such as esters of phthalic acid, and aliphatic and aromatic hydrocarbons and mixtures of such hydrocarbons, examples of which include hexane, mineral oil, benzene, toluene, xylene and (iso)paraffins. Other suitable solvents will be familiar to one of ordinary skill in the art.